Magma-mush interaction at the base of a post-collisional batholith: field evidence from Capo Vaticano Promontory granitoids (Calabria, southern Italy)

Except in migmatite complexes, well-displayed field evidence of the possible physical behaviour of magma-crystal mush-solid rock systems is quite uncommon. Tonalites from Capo Vaticano Promontory, making up the oldest and deepest granitoid unit of the c. 13 km-thick late Variscan Serre Batholith, show outstanding examples of magma-mush interaction, at the transition with both underlying migmatite host rocks and overlying porphyritic granodiorites. Basal tonalites exhibit varying stages of interaction with garnet-bearing or garnet-free granitic magma produced by melting of the metapelitic migmatites after tonalite intrusion. Initial stages are characterized by infiltration of the granitic magma through the mushy tonalite along irregular channels, then forming an interconnected network of cm- to dm-wide channels. In some outcrops with high volumetric proportions of granitic magmas, peculiar hybrid rocks, characterized by pervasive intermingling of granite and tonalite, occur as the result of cm-scale disaggregation of the mushy tonalites.  In other outcrops, evidence of interaction between a crystal-poor tonalitic mush and the granitic magma is only provided by the occurrence of large peritectic garnet in apparently homogeneous tonalite. At the roof of the tonalitic unit, emplacement of the overlying porphyritic granodioritic magma involved displacement of the mushy tonalite, with local disaggregation in rounded blocks up to 1.5 meter in size. Compared to the basal tonalites, such evidence indicates a more rigid state of the roof tonalites at the time of granodiorite emplacement, even though rare occurrence of hybrid rocks testifies for possible mixing processes also between the granodioritic magma and mushy tonalite. Finally, an outstanding field evidence of the physical behaviour of mush systems, with significant implications for the mechanisms of magma differentiation, is given by mechanical accumulation of K-feldspar megacrysts at a place where the granodioritic magma was intruding the mushy tonalite, but only the liquid part of the magma was able to pass through, depicting a clear filter-press mechanism. This preliminary work aims to contribute to the general understanding of the evolution of mushy regions by presenting some outstanding examples of magma-mush interactions taking place during the construction of the deep-intermediate levels of the Serre Batholith (depth of c. 20-17 km depth), as a base for further in-depth multidisciplinary investigations.